EP0249640B1 - Multi-channel stereo reproducing apparatus - Google Patents

Abstract

A multi-channel stereo reproducing apparatus which is capable of favorably reproducing sounds from both a recording medium in which are recorded multi-channel stereo signals of which high-frequency components of rear signals are pre-emphasized and a recording medium in which are recorded ordinary two-channel stereo signals based upon a so-called DolbyR system, such as an audio disk or the like, so that pictures and sounds reproduced, for example, in a theatre or a cinema house can be enjoyed at home. If a left front signal is denoted by LF, a right front signal by RF, a central front signal by F, and a rear signal by B, these signals are encoded into a left composite signal LT = LF + F/2 + B/2 and a right composite signal RT = RF + F/2 - B/2, and high-frequency components of the rear signal B are pre-emphasized and are recorded onto a recording medium that is to be reproduced by the multi-channel stereo reproducing apparatus. During the reproduction of sounds from the recording medium in which are recorded two-channel stereo signals consisting of right and left front signals RF and LF only in which neither central front signal F nor rear signal B have been encoded, a de-emphasis circuit is by-passed.

Description

• This invention relates to multi-channel stereo reproducing apparatus.
• When a film is shown in a cinema, the audio signal is generally reproduced by means of a Dolby surround-phonic 4-channel stereo reproducing system, so as to produce realistic sound.
• When a cinema film is made, a video signal is recorded, and at the same time left front sound, right front sound, centre front sound and rear sound are respectively recorded. In this case, a left front signal L_F, a right front signal R_F, a centre front signal F and a rear signal B are respectively encoded and recorded. To improve the signal-to-noise (S/N) ratio of the rear signal B, there is employed a so-called modified B-type Dolby system, in which a high frequency component of frequency higher than 1 kHz is emphasized by a predetermined amount. Figure 3 shows a frequency vs. response characteristics where the recording level is at 0 dB, -10 dB, -20 dB and -30 dB in this modified B-type Dolby system.
• The left front signal L_F, the right front signal R_F, the centre front signal F and the rear signal B are then further encoded to form a left composite signal L_T and a right composite signal R_T which are respectively expressed as ${\text{L}}_{\text{T}}{\text{=L}}_{\text{F}}\text{+F/2+B/2}$

  

  and ${\text{R}}_{\text{T}}{\text{=R}}_{\text{F}}\text{+F/2-B/2}$

  

  . These signals L_T and R_T are optically recorded, in the case of, for example, a 35mm film, at a sound track portion located at a predetermined position of the film, using a brightness change system which carries out the recording on the basis of change of light and shade. When the film is shown, the signals L_T and R_T thus optically recorded are decoded so as to produce the left composite signal L_T, the right composite signal R_T, a sum signal L_T+R_T, and a difference signal L_T-R_T. These four signal are reproduced by a left front loudspeaker, a right front loudspeaker, a centre front loudspeaker and a rear loudspeaker. In this case, since the high frequency component of the rear signal B has been pre-emphasized by the modified B-type Dolby system as described above, upon reproducing, the high frequency component of the difference signal L_T-R_T is attenuated (de-emphasized) so as to make the total frequency characteristic flat relative to the rear signal B. When the audio signal is reproduced by such a 4-channel stereo reproducing system, the sound effect is enhanced and realistic.
• Magnetic tapes and video discs of the screen music or the motion picture itself are also available on the market. In this case, since the signals recorded on the magnetic tape and the video disc are the composite signals L_T and R_T as described above, there has been proposed a multi-channel stereo reproducing apparatus illustrated in Figure 1.
• The apparatus comprising left and right composite signal input terminals 1 and 2 to which the left and right composite signals L_T and R_T are supplied. The terminals 1 and 2 are connected to the input side of a balancing circuit 3 which produces left and right composite signals L_T and R_T which are so balanced as to localize an acoustic image normally.
• The outputs of the balancing circuit 3 are connected through amplifiers 4 and 5 to left and right front loudspeakers 6 and 7 which respectively reproduce the left and right composite signals L_T and R_T supplied to the terminals 1 and 2.
• Also, the outputs of the balancing circuit 3 are connected to the input of a mixing circuit 8 which generates the sum signal L_T+R_T. The output of the mixing circuit 8 is connected through an amplifier 9 to a centre front loudspeaker 10 which reproduces the sum signal L_T + R_T developed at the output of the mixing circuit 8.
• Further, the outputs of the balancing circuit 3 are connected to the input of a subtracting circuit 11 which generates the left and right composite signals L_T and R_T from which the monaural components are removed. The output of the subtracting circuit 11 is connected to the input of a delay circuit 12 which generates at its output side a delayed difference signal Δt(L_T-R_T), delayed by a predetermined time. Thus, a front localization impression and a sound field widening impression are obtained. The output of the delay circuit 12 is connected to the input of a low-pass filter 13 for passing therethrough a signal of a low frequency band, and the filter 13 generates at its output the delayed difference signal Δt(L_T-R_T) in which a clock pulse component resulting from passing the signal through the delay circuit 12 has been removed, and frequencies higher than a predetermined frequency are removed, so as to balance the low frequency component which is deficient in the difference signal L_T-R_T. The output of the low-pass filter 13 is connected to the input of a de-emphasis circuit 14 which attenuates the high frequency component of the delayed difference signal Δt(L_T-R_T) higher than 1 kHz, thus producing a delayed difference signal Δt(L_T-R_T) in which the total frequency characteristic of the rear signal B of which the high frequency component higher than 1 kHz has been emphasized is made flat. The output of the de-emphasis circuit 14 is connected through amplifiers 15 and 16 to left and right rear loudspeakers 17 and 18 by which the delayed difference signal Δt(L_T-R_T) in which the frequency characteristic of the rear signal B is made flat, is reproduced.
• In this apparatus, the left and right composite signals L_T and R_T are reproduced from the left and right front loudspeakers 6 and 7, the sum signal L_T+R_T is reproduced from the centre front loudspeaker 10 and the delayed difference signal Δt(L_T-R_T) is reproduced from the left and right rear loudspeakers 17 and 18. Consequently, the sound effect is enhanced to produce good stereo presence.
• However, with this apparatus, when reproducing a recording medium, such as a record disc or a cassette tape on which only the standard 2-channel stereo signal formed of the left front signal L_F and the right front signal R_F are recorded, since the delayed difference signal Δt(L_F+R_F) from the delay circuit 12 passes through the de-emphasis circuit 14, its high frequency component higher than 1 KHz is attenuated. Then, the delayed difference signal Δt(L_F-R_F) in which the high frequency component has been attenuated is reproduced from the left and right rear loudspeakers 17 and 18, so that the high frequency component of the sound reproduced by the rear loudspeakers is attenuated unnaturally. Thus, satisfactory stereo presence cannot be obtained.
• A different sound system for cinema use is disclosed in Japanese patent specification JP-A-54/62801 (GB-A-2 006 583), while Japanese patent specification JP-A-49/120201 discloses a low-pass filter which can be selectively by-passed in a 2-channel or 4-channel stereo mode apparatus.
• According to the present invention there is provided a multi-channel stereo reproducing apparatus for reproducing a recording medium wherein a left front signal L_F, a right front signal R_F, a centre front signal F and a rear signal B have been encoded to form left and right composite recorded signals L_T and R_T, where ${\text{L}}_{\text{T}}{\text{=L}}_{\text{F}}\text{+F/2+B/2}$

  

  and ${\text{R}}_{\text{T}}{\text{=R}}_{\text{F}}\text{+F/2-B/2}$

  

  , and where the high frequency component of said rear signal B has been pre-emphasized, the apparatus comprising:
  a reproducing circuit for reproducing said left composite signal L_T, said right composite signal R_T and a sum signal L_T+R_T; and
  a reproducing circuit for reproducing a difference signal L_T-R_T through a delay circuit and a de-emphasis circuit;
  characterized by:
  means for by-passing said de-emphasis circuit when reproducing a recording medium on which a 2-channel stereo signal formed of only said left and right front signals L_F and R_F is recorded.
• The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
• Figure 1 is a block diagram of a previously proposed multi-channel stereo reproducing apparatus;
• Figure 2 is a block diagram of an embodiment of multi-channel stereo reproducing apparatus according to the present invention; and
• Figure 3 is a frequency vs. response graph.
• In the embodiment shown in Figure 2, the output of the low-pass filter 13 is connected to the input of the de-emphasis circuit 14 and to one fixed contact 19a of a change-over switch 19. In this case, the de-emphasis circuit 14 is modified to de-emphasize the high frequency component of the audio signal pre-emphasized by the modified B-type Dolby system, thereby to make the total frequency characteristic flat. The frequency characteristic of the de-emphasis circuit 14 may be such that when the signal level is about 30 dB, a signal level of a signal higher than about 4 kHz is emphasized by 5dB.
• The output of the de-emphasis circuit 14 is connected to the other fixed contact 19b of the switch 19, and a movable contact 19c thereof is connected through the amplifiers 15 and 16 to the left and right rear loudspeakers 17 and 18, whereby the de-emphasis circuit 14 can be by-passed, if necessary. Otherwise the arrangement is as shown in Figure 2.
• In this embodiment, when the movable contact 19c engages the fixed contact 19b, the circuit arrangement is effectively the same as that of Figure 2. Accordingly, the left front signal L_F, the right front signal R_F, the centre front signal F and the rear signal B have been encoded respectively to form the left and right composite signals L_T and R_T which are respectively expressed as ${\text{L}}_{\text{T}}{\text{=L}}_{\text{F}}\text{+F/2+B/2}$

  

  and ${\text{R}}_{\text{T}}{\text{=R}}_{\text{F}}\text{+F/2-B/2}$

  

  . Thus when reproducing from a recording medium on which a high frequency component of the rear signal B higher than 1 kHz has been pre-emphasized by the modified B-type Dolby system and recorded; for example, a video disc on which screen music recorded for a film is recorded, the left composite signal ${\text{L}}_{\text{T}}{\text{= L}}_{\text{F}}\text{+F/2+B/2}$

  

  is reproduced from the left front loudspeaker 6, the right composite signal ${\text{R}}_{\text{T}}{\text{=R}}_{\text{F}}\text{+F/2-B/2}$

  

  is reproduced from the right front loudspeaker 7, the sum signal ${\text{L}}_{\text{T}}{\text{+R}}_{\text{T}}{\text{=L}}_{\text{F}}{\text{+R}}_{\text{F}}\text{+F}$

  

  of the left composite signal L_T and the right composite signal R_T is reproduced from the centre front loudspeaker 10, and the delayed difference signal ${\text{Δt(L}}_{\text{T}}{\text{-R}}_{\text{T}}{\text{) = Δt(L}}_{\text{F}}{\text{-R}}_{\text{F}}\text{+B)}$

  

  is reproduced from the left and right rear loudspeakers 17 and 18. In this case, since the delayed difference signal ${\text{Δt(L}}_{\text{T}}{\text{-R}}_{\text{T}}{\text{)= Δt(L}}_{\text{F}}{\text{-R}}_{\text{F}}\text{+B)}$

  

  

  is reproduced such that its high frequency component is attenuated by the de-emphasis circuit 14 and such that its total frequency characteristic relative to the rear signal B is made flat, the sound effect can be enhanced to produce stereo presence as obtained in a concert hall or cinema.
• With this embodiment, it is possible to reproduce the standard 2-channel stereo signal formed only of the left and right front signal L_F and R_F, in which the centre front signal F and the rear signal B are not present. In this case, when the left and right front signals L_F and R_F are supplied to the left and right composite signal input terminals 1 and 2, the left and right front signals L_F and R_F are reproduced from the left and right loudspeakers 6 and 7, the sum signal L_F+R_F is reproduced from the centre front loudspeaker 10, and the delayed difference signal Δt(L_F-R_F) is reproduced from the rear loudspeakers 17 and 18. In addition, since the movable contact 19c engages one fixed contact 19a to by-pass the de-emphasis circuit 14, thereby directly connecting the output of the low-pass filter 13 to the amplifiers 15 and 16, without attenuating the high frequency component of the delayed difference signal t(L_F-R_F) obtained at the output of the low-pass filter 13, this delayed difference signal Δt(L_F-R_F) can be reproduced. When the delayed difference signal Δt(L_F-R_F) is reproduced together with the left front signal L_F, the right front signal R_F and the sum signal L_F+R_F, it is well known that stereo presence can be increased and hence the atmosphere of a concert hall or cinema can be obtained.
• While in the above embodiment, the invention is applied to a multi-channel stereo reproducing apparatus for reproducing a recording medium on which a multi-channel stereo signal in which the rear signal B has been pre-emphasized by the modified B-type Dolby system is recorded, the invention is not limited to this, but can be applied to a multi-channel stereo reproducing apparatus for reproducing a recording medium on which a signal pre-emphasized by another noise reduction system has been recorded, or an apparatus in which the above delay circuit is formed of a digital circuit comprising an A/D converter and a D/A converter.

Claims (5)

1. A multi-channel stereo reproducing apparatus for reproducing a recording medium wherein a left front signal L_F, a right front signal R_F, a centre front signal F and a rear signal B have been encoded to form left and right composite recorded signals L_T and R_T, where ${\text{L}}_{\text{T}}{\text{=L}}_{\text{F}}\text{+F/2+B/2}$

   

   and ${\text{R}}_{\text{T}}{\text{=R}}_{\text{F}}\text{+F/2-B/2}$

   

   , and where the high frequency component of said rear signal B has been pre-emphasized, the apparatus comprising:
   a reproducing circuit (3, 8) for reproducing said left composite signal L_T, said right composite signal R_T and a sum signal L_T+R_T; and
   a reproducing circuit (3, 11) for reproducing a difference signal L_T-R_T through a delay circuit (12) and a de-emphasis circuit (14);
   characterized by:
   means (19) for by-passing said de-emphasis circuit (14) when reproducing a recording medium on which a 2-channel stereo signal formed of only said left and right front signals L_F and R_F is recorded.
2. Apparatus according to claim 1 wherein said by-pass means (19) comprises a change-over switch (19) having a pair of fixed contacts (19a, 19b) and a movable contact (19c), said pair of fixed contacts (19a, 19b) being respectively connected to the input and output sides of said de-emphasis circuit (14), and said movable contact (19c) being connected to the output side of said reproducing circuit (3, 11).
3. Apparatus according to claim 2 wherein said de-emphasis circuit (14) has a frequency characteristic in which a signal level of a high frequency is emphasized in response to a signal level of said difference signal L_T-R_T.
4. Apparatus according to claim 3 wherein said frequency characteristic is such that when the signal level is about 30 dB, a signal level of a signal higher than about 4 kHz is emphasized by 5 dB.
5. Apparatus according to claim 1 wherein said delay circuit (12) comprises a digital circuit having an A/D converter and a D/A converter.

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